A flash memory device may be a device that electrically writes and erases data. The flash memory device may be a device that stores electric charges in floating gates or discharges the electric charges stored in the floating gates to a substrate, using tunneling of the electric charges through a tunnel insulation layer.
The tunneling, which may an important operation of the flash memory device, depends upon how much voltage applied to a control gate electrode may be transmitted to the tunnel insulation layer to create an electric field. The ratio of the voltage applied to the tunnel insulation layer to the voltage applied to the control gate electrode may be represented by a coupling ratio. The coupling ratio may be increased when the capacitance between the floating gates and the control gate may be greater than the capacitance between the substrate and the floating gates.
A silicon oxide-silicon nitride-silicon oxide (ONO) layer having a high dielectric constant may be used as an intergate dielectric layer to increase the capacitance between the floating gates and the control gate. As semiconductor devices have become more highly integrated, intergate dielectric layers having a higher dielectric constant have become more desirable. If the dielectric constant is too high, however, leakage current characteristics may be deteriorated, and may cause retention characteristics to be reduced.
A method of increasing opposite areas of the floating gates and the control gate also has been proposed to increase the capacitance between the floating gates and the control gate. If a concavo-convex structure is formed on the top of the floating gates, it may be possible to increase the area of the intergate dielectric layer formed on the floating gates.
FIGS. 1 and 2 are drawings illustrating a related art method of fabricating a flash memory device.
Referring to FIG. 1, device isolation layer 12 may be formed on semiconductor substrate 10 and may define active regions 14, and floating gate layer 18 may be formed on active region 14 via tunnel insulation layer 16. Photoresist patterns 20 may be formed on floating gate layer 18. Photoresist patterns 20 may be formed by exposing the photoresist film using reticle 22 having mask patterns 24 formed thereon. At this time, it may be possible to reduce the intensity of light transmitted through regions between mask patterns 24, by adjusting a width of mask patterns and the distance between the mask patterns. As a result, the exposure degree of the photoresist film may be changed, and therefore, photoresist patterns 20 may be formed on floating gate layer 18 in a concavo-convex structure.
Referring to FIG. 2, photoresist patterns 20 and floating gate layer 18 may be anisotropically etched. Since photoresist patterns 20 may have a concavo-convex structure, the concavo-convex structure of photoresist patterns 20 may be transferred to floating gate layer 18, thereby forming floating gate patterns 18a having a concavo-convex structure.
In the related art, the height difference between the concave parts and the convex parts of floating gate patterns 18a and the width of the opposite concave parts of floating gate patterns 18a may depend upon the shape of photoresist patterns 20. As described above, photoresist patterns 20 may have the concavo-convex structure due to the intensity difference of the exposure according to the width of the mask patterns formed on reticle 22 and the distance between the mask patterns formed on reticle 22. The arrangement of mask patterns 24 may be chosen to obtain a desired shape of mask patterns 24.
Consequently, an adjustment of the size of floating gate patterns 18a, the height difference between the concave parts and the convex parts of floating gate patterns 18a and a width of the concave and convex parts of floating gate patterns 18a may be limited.
Also, a complicated reticle manufacturing process may be required to obtain desired floating gate patterns 18a, which may result in an inability to rapidly change the structure of the device. In addition, the characteristics of photoresist film supplied to a production line may be variable, with the result that the shape of photoresist patterns 20 may be different from what may be desired.